Optical film and method of preparing same

ABSTRACT

The present invention relates to an optical film and method of manufacturing the same. The optical film of the present invention includes an acrylic resin and a core-shell type graft copolymer wherein the core includes a conjugate diene rubber, and the shell includes an acrylic monomer, an aromatic vinyl monomer, and a maleimide monomer.

TECHNICAL FIELD

The present invention relates to an optical film and a method ofmanufacturing the same. More specifically, the present invention relatesto an optical film that has excellent impact resistance, heatresistance, and the like, and a method of preparing the same. Theoptical film may be usefully applied to an electronic device such as adisplay device including an LCD.

This application claims priority to Korean Patent Application No.10-2008-0097400 filed on Oct. 2, 2008 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND ART

In recent years, in accordance with the advance in optical technology,various types of display technologies such as plasma display panels(PDP), liquid crystal displays (LCD), organic/inorganicelectroluminescent displays (ELD) and the like have been suggested andsold in the market instead of a known cathode-ray tube. In theabove-mentioned displays, the use of various types of plastic films hasbeen suggested and the required characteristics thereof have beensophisticated. For example, in the case of liquid crystal displays, inorder to obtain product slimness and lightness as well as to improvedisplay characteristics, various types of plastic films are used in apolarizing plate, a retardation film, a plastic substrate, a light guideplate, and the like.

In general, the polarizing plate has a structure in which the triacetylcellulose film (hereinafter, referred to as TAC film) as a protectivefilm is laminated by using an aqueous adhesive made of a polyvinylalcohol aqueous solution on a polarizer. However, both the polyvinylalcohol film used as the polarizer and the TAC film used as theprotective film for a polarizer have poor heat and humidity resistance.Therefore, if a polarizing plate which is made of the films is used in ahigh temperature or high humidity atmosphere for a long period of time,the degree of polarization may be reduced, the polarizer and theprotective film may be separated from each other, or optical propertiesmay be reduced. Thus, in terms of the purposes, there are variouslimits. In addition, in the case of the TAC film, a change in in-planeretardation (R_(in)) and thickness retardation (R_(th)) is significantaccording to a change in the ambient temperature/humidity atmosphere. Inparticular, a change in retardation in respect to incident light at aninclined direction is significant. If a polarizing plate that includes aTAC film having the above-mentioned characteristics as a protective filmis applied to a liquid crystal display, the viewing anglecharacteristics are changed according to the change in the ambienttemperature/humidity of the surrounding atmosphere, thus reducing imagequality. In addition, in the case of the TAC film, a change in dimensionis significant according to the change in the ambienttemperature/humidity of the surrounding atmosphere and the photoelasticcoefficient is relatively high, thus, after evaluation in respect todurability in a heat resistant and humidity resistant atmosphere, achange in retardation characteristics occurs locally, thereby easilyreducing the image quality.

As a material used to avoid the disadvantages of the TAC film, a(meth)acrylic resin is well known. However, it is known that(meth)acrylic resin is easily broken or split, causing problems in termsof transportability during the production of the polarizer plate andthat the productivity thereof is poor. In addition, when an acrylateresin is used as a material of a film, it is necessary that a castingprocess be used, and thus there are problems in that it is difficult toperform the process and the costs are high.

In the case of the retardation film, the film that is made of a styreneresin is a material that shows optical anisotropic properties that therefractive index is increased in a direction perpendicular to thealignment direction when the film is stretched and aligned, and it isknown that the film is stretched to be used for manufacturing the filmhaving the positive thickness retardation (R_(th)). In addition, thereare advantages in that the styrene resin has excellent economicefficiency and transparency. However, there are problems in that theheat resistance is insufficient and the mechanical properties are poorexcept for when costly special monomers are used together to perform themanufacturing process.

In addition, a biaxially-stretched polycarbonate film or a filmincluding a uniaxially-stretched cyclic olefin polymer (COP) on whichliquid crystal is coated is used as a compensation film or a retardationfilm. However, in the case of the biaxially-stretched polycarbonatefilm, there are problems in that the retardation varies depending on awavelength of incident light, leading to imperfect retardationcompensation. In the case of the liquid crystal coated COP, there areproblems in that the price is high and the processes are complex.

Although an acrylic copolymer that has excellent heat resistance andtransparency has been studied in order to solve the above-mentionedproblems, sufficient impact resistance cannot be attained by using onlythe acrylic copolymer, and the copolymer may be easily split or broken.Thus, various types of tenacity modifiers are added to secure impactresistance. In this case, if a large amount of a tenacity modifier isused for sufficient impact resistance, heat resistance and transparencyare reduced. If the molecular weight of the tenacity modifier isincreased, it is difficult to produce and process the copolymer.

DISCLOSURE Technical Problem

An aspect of the present invention provides an optical film that can beeasily manufactured by using an extrusion process at low cost and hasexcellent impact resistance and heat resistance, and a method ofmanufacturing the same. In the case of the optical film, since theoptical isotropic properties are excellent before stretching, the filmcan be used for various types of purposes such as a polarizing plateprotective film, and after stretching, since it is possible to provideuniform and stable retardation, the film can be applied to a retardationfilm.

Technical Solution

According to an aspect of the present invention, there is provided anoptical film including: an acrylic resin and a core-shell type graftcopolymer wherein the core includes a conjugate diene rubber, and theshell includes an acrylic monomer, an aromatic vinyl monomer, and amaleimide monomer.

According to another aspect of the present invention, there is provideda method of manufacturing an optical film, the method including:preparing a resin composition including an acrylic resin and acore-shell type graft copolymer wherein the core includes a conjugatediene rubber, and the shell includes an acrylic monomer, an aromaticvinyl monomer, and a maleimide monomer, and using the resin compositionto form the film.

According to another aspect of the present invention, there is provideda retardation film including: an acrylic resin and a core-shell typegraft copolymer wherein the core includes a conjugate diene rubber, andthe shell includes an acrylic monomer, an aromatic vinyl monomer, and amaleimide monomer.

According to another aspect of the present invention, there is provideda method of manufacturing a retardation film, the method including:preparing a resin composition including an acrylic resin and acore-shell type graft copolymer wherein the core includes a conjugatediene rubber, and the shell includes an acrylic monomer, an aromaticvinyl monomer, and a maleimide monomer, using the resin composition toform the film, and uniaxially or biaxially stretching the film.

According to another aspect of the present invention, there is provideda retardation film having an in-plane retardation value of about 100 nmto about 120 nm, represented by Mathematical Formula 1 and a thicknessretardation value of about 20 nm to about 60 nm, represented byMathematical Formula 2.

R_(in)=(n _(x) −n _(y))×d  Mathematical Formula 1

R_(th)=[(n _(x) +n _(y))/2−n ₂ ]×d  Mathematical Formula 2

In Mathematical Formulas 1 and 2, n_(x) is a refractive index in adirection in which the index is largest, in the film plane,n_(y) is a refractive index in a direction perpendicular to the n_(x)direction, in the film plane,n_(z) is a refractive index in a thickness direction, andd is film thickness.

According to another aspect of the present invention, there is provideda polarizing plate including the optical film.

According to another aspect of the present invention, there is providedan electronic device including the optical film or retardation film.

Advantageous Effects

An optical film according to the present invention may replacerelatively expensive TAC resins that have conventionally been used sincethe film has excellent impact resistance and heat resistance, theoptical isotropic properties are excellent before the stretching, andthus the film can be used for various types of purposes such as apolarizing plate protective film. After the film is stretched, it ispossible to provide uniform and stable retardation, and thus the filmmay be used as a retardation film. In addition, since the optical filmaccording to the present invention can be easily manufactured by usingan extrusion process at low cost, the film has better effects than afilm that is made only of a conventional acrylate resin.

BEST MODE

Exemplary embodiments of the present invention will now be described indetail.

An optical film according to the present invention includes an acrylicresin and a core-shell type graft copolymer wherein the core includes aconjugate diene rubber, and the shell includes an acrylic monomer, anaromatic vinyl monomer, and a maleimide monomer.

In the optical film according to the present invention, an acrylic resinwell known in the art may be used, and in particular, a homo- orcopolymer of an acrylic monomer; a copolymer of an acrylic monomer andan aromatic vinyl monomer; or a copolymer of an acrylic monomer, anaromatic vinyl monomer, an acid anhydride, and the like may be used.

It is more preferable to use an acrylic monomer, an aromatic vinylmonomer, and an acid anhydride as the acrylic resin in 1).

The acrylic monomer may be a compound having a double bond betweenconjugated carbons and carbonyl group of an ester group, and asubstituent group thereof is not specifically limited. In thespecification, it is to be understood that the term “acrylic monomer”refers to acrylate and an acrylate derivative, and also includes alkylacrylate, alkyl methacrylate, alkyl butacrylate and the like. Forexample, examples of the acrylic monomer include a compound that isrepresented by the following Formula 1.

In Formula 1,

R₁, R₂, and R₃ each independently represent a hydrogen atom or amonovalent hydrocarbon group having 1 to 30 carbon atoms with or withouta heteroatom, at least one of R₁, R₂ and R₃ may be an epoxy group, andR₄ represents a hydrogen atom or an alkyl group having 1 to 6 carbonatoms.

Specifically, the acrylic monomer may be at least one selected from thegroup consisting of methylacrylate, ethylacrylate, methylmethacrylate,ethylmethacrylate, methylethacrylate, and ethylethacrylate, and inparticular, it is most preferable that methylmethacrylate (MMA) may beused.

The aromatic vinyl monomer may be a monomer having a structure, in whicha benzene core is substituted or unsubstituted with one or more C₁ to C₅alkyl groups or halogen groups. For example, the monomer may be at leastone styrene monomer selected from the group consisting of styrene,α-methylstyrene, p-methylstyrene, and vinyltoluene.

The acid anhydride may be a carboxylic acid anhydride, and a monovalent-or a polyvalent (including bivalent or more) carboxylic acid anhydride.A maleic acid anhydride or derivatives thereof may be used, and forexample, a compound represented by the following Formula 2 may be used.

In Formula 2,

R₇ and R₈ each independently represent a hydrogen atom or an alkyl grouphaving 1 to 6 carbon atoms.

When a copolymer of an acrylic monomer, an aromatic vinyl monomer, andan acid anhydride is used as the acrylic resin in 1), the weight ratioof each monomer is preferably (60˜98):(1˜45):(1˜15).

The acrylic monomer may contribute to optical properties, the aromaticvinyl monomer may contribute to moldability and retardation of the film,and the acid anhydride may contribute to heat resistance.

The acrylic resin may be polymerized by using a method known in the art,for example, a bulk polymerization method.

The copolymers may further include a maleimide monomer as an additionalcomonomer. Examples of the maleimide monomer may includephenylmaleimide, cyclohexylmaleimide, and the like. The comonomer ispreferably contained in an amount of about 5 parts or less by weightbased on 100 parts by weight of the copolymer. A maleimide monomer maybe further included as the comonomer to further enhance compatibilitywith a core-shell type graft copolymer having an increased heatresistance in the shell.

The acrylic resin may have a glass transition temperature of about 110°C. or more, preferably about 115° C. or more, a weight average molecularweight of about 100,000 to about 180,000, a haze of about 2.0% or less,a melt index (MI) of about 2 to about 10, and a refractive index ofabout 1.490 to about 1.520.

The optical film according to the present invention is characterized inthat it includes a core-shell type graft copolymer including a conjugatediene rubber component in addition to an acrylic resin.

Since stability under UV light and heat resistance are required for filmmaterials used in an image display device such as an LCD, it is knownthat conjugate diene compounds including a double bond are not used.However, the conjugate diene rubber component may be used in the form ofa copolymer having a specific structure, and at the same time componentsof the copolymer and compositions thereof may be controlled to enhancethe toughness of the film according to the type of the conjugate dienecompound and solve the deterioration of stability and heat resistance,which is known as a limitation in conventional conjugate dienecompounds.

In the optical film according to the present invention, the core of thecore-shell type graft copolymer includes a conjugate diene rubber.

Examples of the conjugate diene rubber may include an ethylene-propylenediene rubber, a styrene-butadiene rubber, a butadiene rubber, and thelike, and it is more preferable to use a butadiene rubber.

The core of the core-shell type graft copolymer may use a conjugatediene rubber only, and a copolymer of a conjugate diene rubber and anacrylic monomer.

The conjugate diene rubber is preferably contained in an amount of about10 to about 60 parts by weight based on 100 parts by weight of thecore-shell type graft copolymer in 2). When an acrylic monomer is addedto the core, the acrylic monomer is preferably contained in an amount ofabout 10 parts or less by weight based on 100 parts by weight of thecore-shell type graft copolymer.

The shell of the core-shell type graft copolymer may use a copolymerincluding an acrylic monomer, an aromatic vinyl monomer, and a maleimidemonomer.

In the copolymer including an acrylic monomer, an aromatic vinylmonomer, and a maleimide monomer, the weight ratio of each monomer ispreferably (38˜88):(1˜30):(1˜10).

Particularly, in the optical film according to the present invention, amaleimide monomer may be contained in an amount of about 1 to about 10parts by weight, preferably about 3 to about 7 parts by weight, based on100 parts by weight of the copolymer including an acrylic monomer, anaromatic vinyl monomer, and a maleimide monomer to increase the glasstransition temperature of the shell. Accordingly, the glass transitiontemperature of the optical film may be increased without greatlyincreasing the molecular weight of the shell.

When the film contains the maleimide monomer in an amount of less thanabout 1 part by weight, the film may not have sufficient heatresistance. When the maleimide monomer is contained at more than about10 parts by weight, the graft ratio may be decreased and compatibilitywith the acrylic resin may be deteriorated to generate haze when thefilm is extruded.

Examples of each monomer constituting the shell are the same as examplesof components of the acrylic resin.

The shell of the core-shell type graft copolymer may further include anacrylonitrile monomer as an additional comonomer. The acrylonitrilemonomer may be at least one selected from the group consisting ofacrylonitrile, methacrylonitrile, and ethacrylonitrile.

The acrylonitrile monomer is preferably contained in an amount of about10 parts or less by weight based on 100 parts of a copolymer includingan acrylic monomer, an aromatic vinyl monomer, and a maleimide monomer.

The core-shell type graft copolymer uses a copolymer including aconjugate diene rubber, an acrylic monomer, an aromatic vinyl monomer,and a maleimide monomer, and may be graft-polymerized to have acore-shell structure by a method known in the art. For example, atypical emulsion polymerization method may be used. Here, the graftratio is preferably about 30% to about 80%.

The core-shell type graft copolymer is characterized in that it has aweight average molecular weight of about 80,000 to about 300,000,preferably about 80,000 to about 200,000. When weight average molecularweight is more than about 300,000, it may be helpful in heat resistance,but there may be problems in production and processing.

In the optical film according to the present invention, the weight ratioof the acrylic resin and the core-shell type graft copolymer ispreferably (60˜99):(1˜40).

In the optical film according to the present invention, the thickness ofthe optical film is preferably about 20 μm to about 100 μm, but it isnot limited thereto.

In addition, the optical film may have a glass transition temperature ofabout 100° C. or more, a coefficient of thermal expansion (CTE) of about120 or less, and a haze of about 2.0% or less.

Since the optical film according to the present invention has excellentproperties such as glass transition temperature, CTE, haze, and thelike, it can be seen that transparency, heat resistance, opticalisotropic properties, and the like are excellent. Accordingly, the filmmay be used for various purposes, such as a polarizing plate protectivefilm and the like to replace an expensive TAC film which has beenconventionally used.

In addition, a method of manufacturing an optical film according to thepresent invention includes preparing a resin composition including anacrylic resin and a core-shell type graft copolymer wherein the coreincludes a conjugate diene rubber, and the shell includes an acrylicmonomer, an aromatic vinyl monomer, and a maleimide monomer, and usingthe resin composition to form the film.

In the method of preparing the optical film according to the presentinvention, a general additive, for example, a plasticizer, a lubricant,an impact modifier, a stabilizing agent, an ultraviolet ray absorptionagent, and the like may be added to the resin composition.

In particular, when the optical film according to the present inventionis used as a protective film of a polarizer, in order to protect thepolarizer and the liquid crystal panel from external ultraviolet rays,an ultraviolet ray absorption agent may be added to the copolymer resin.Examples of the ultraviolet ray absorption agent may include, but arenot specifically limited to, a benzotriazole ultraviolet ray absorptionagent and a triazine ultraviolet ray absorption agent. A hindered aminelight stabilizer such as bis(2,2,6,6-tetramethyl-4-piperidyl)sebaceatemay be used. Tinuvin 328, Tinuvin 321, and Tinuvin 360 may be used.Igafos 168, Iganox 1076, Iganox 245, and the like may be added as athermal stabilizing agent.

In the method of manufacturing the optical film according to the presentinvention, a method of forming the film may be one known in the art.Specifically, a casting method, an extrusion process, and the like maybe used.

In addition, the present invention provides a retardation film includingan acrylic resin and a core-shell type graft copolymer wherein the coreincludes a conjugate diene rubber, and the shell includes an acrylicmonomer, an aromatic vinyl monomer, and a maleimide monomer.

In the retardation film according to the present invention, sinceinformation on the acrylic resin, the conjugate diene rubber, theacrylic monomer, the aromatic vinyl monomer, the maleimide monomer, andthe core-shell type copolymer is the same as what is described above, aspecific description thereof will be omitted.

The retardation film according to the present invention may have anin-plane retardation value of about 100 nm to about 120 nm, representedby Mathematical Formula 1 and a thickness retardation value of about 20nm to about 60 nm, represented by Mathematical Formula 2, but it is notlimited thereto.

Since the retardation film according to the present invention mayprovide uniform and stable in-plane retardation and thicknessretardation, it may be used as a retardation film in various liquidcrystal mode image display devices.

In particular, the retardation film according to the present inventionmay be applied to an IPS mode image display device, but it is notlimited thereto.

In addition, the method of manufacturing the retardation film accordingto the present invention includes preparing a resin compositionincluding an acrylic resin and a core-shell type graft copolymer whereinthe core includes a conjugate diene rubber, and the shell includes anacrylic monomer, an aromatic vinyl monomer, and a maleimide monomer,using the resin composition to form the film, and uniaxially orbiaxially stretching the film.

In the method of manufacturing the retardation film according to thepresent invention, the stretching may be performed at a temperaturerange of preferably (T_(g)-30° C.) to (T_(g)+30° C.) and more preferably(T_(g)−30° C.) to (T₉+20° C.) based on the glass transition temperature(T_(g)) of the resin composition. In addition, the stretching speed andthe stretching ratio may be appropriately controlled to be within arange capable of achieving the object of the present invention.

The optical film according to the present invention may be used as apolarizer protective film. In this case, the surface may be modified inorder to improve the adhesion strength thereof. Examples of themodifying method include a method of treating a surface of theprotective film by using corona treatment, plasma treatment, UVtreatment, and the like, and a method of forming a primer layer on thesurface of the protective film. Both the methods may be simultaneouslyused. The type of primer is not specifically limited, but it ispreferable to use a compound having a reactive functional group such asa silane coupling agent.

A polarizing plate including the optical film according to the presentinvention as a protective film includes a polarizer and a protectivefilm provided on at least one side of the polarizer, and may have astructure in which at least one of the protective films is the opticalfilm according to the present invention.

In the present invention, any polarizer may be used as long as thepolarizer is known in the art, and for example, a film which is made ofpolyvinyl alcohol (PVA) containing iodine or dichromatic dyes may beused. The polarizer may be produced by applying iodine or dichromaticdyes on the PVA film. However, the method of producing the polarizer isnot specifically limited. In the specification, the polarizer does notinclude the protective film, and the polarizing plate includes thepolarizer and the protective film.

The adhesion of the polarizer and the protective film may be performedby using an adhesive layer. Examples of the adhesive which may be usedto combine the protective film and the polarizer are not limited as longas the adhesive is known in the art. Examples of the adhesive include,but are not limited to, a one- or two-liquid type polyvinyl alcohol(PVA) adhesive, a polyurethane adhesive, an epoxy adhesive, astyrene-butadiene rubber (SBR) adhesive, or a hot melt adhesive.

Among the adhesives, a polyvinyl alcohol adhesive may be used. Inparticular, an adhesive that includes a polyvinyl alcohol resincontaining an acetacetyl group and an amine metal compound crosslinkingagent may be used. The adhesive for the polarizing plate may includeabout 100 parts by weight of the polyvinyl alcohol resin containing theacetacetyl group and about 1 to about 50 parts by weight of the aminemetal compound crosslinking agent.

The polyvinyl alcohol resin is not specifically limited as long as theresin is capable of desirably attaching the polarizer and the protectivefilm to each other, and has excellent optical penetration and noconsecutive change such as yellowing. In consideration of a desirablecrosslinking reaction to the crosslinking agent, a polyvinyl alcoholresin containing an acetacetyl group may be used.

The degrees of polymerization and saponification of the polyvinylalcohol resin are not specifically limited as long as the polyvinylalcohol resin contains an acetoacetyl group, but it is preferable thatthe degree of polymerization may be about 200 to about 4,000 and thedegree of saponification may be about 70 mol % to about 99.9 mol %. Inconsideration of a desirable mixing to the contained material accordingto the free movement of molecules, it is more preferable that the degreeof polymerization may be about 1,500 to about 2,500 and the degree ofsaponification may be about 90 mol % to about 99.9 mol %. In connectionwith this, it is preferable that the polyvinyl alcohol resin contain theacetacetyl group in an amount of about 0.1 mol % to about 30 mol %. Inthe above-mentioned range of the acetaceyl group, the reaction to thecrosslinking agent may be desirably performed and the adhesive may havea desired water resistance and adhesion strength.

The amine metal compound crosslinking agent is a water-solublecrosslinking agent that contains a functional group having reactivity tothe polyvinyl alcohol resin, and may be a metal complex containing anamine ligand. Examples of metal that is capable of being applied to themetal complex include a transition metal such as zirconium (Zr),titanium (Ti), hafnium (Hf), tungsten (W), iron (Fe), cobalt (Co),nickel (Ni), ruthenium (Ru), osmium (Os), rhodium (Rh), iridium (Ir),palladium (Pd), platinum (Pt), and the like. Examples of the ligand thatis coupled with the central metal may include any ligand as long as itcontains at least one amine group such as a primary amine, a secondaryamine (diamine), a tertiary amine, or ammonium hydroxide. It ispreferable that the amount of the crosslinking agent used may becontrolled to be within the range of about 1 to about 50 parts by weightbased on 100 parts by weight of the polyvinyl alcohol resin. In theabove-mentioned range, it is possible to provide sufficientlysignificant adhesion strength to a target adhesive and to improve thestorage stability (pot life) of the adhesive.

It is preferable that the pH of the adhesive aqueous solution includingthe polyvinyl alcohol resin containing the acetacetyl group and theamine metal compound crosslinking agent may be adjusted to about 9 orless by using a pH adjusting agent. More preferably, the pH may beadjusted to more than about 2 and about 9 or less, and even morepreferably, about 4 to about 8.5.

The bonding of the polarizer and the protective film may be performedaccording to an attachment method using an adhesive. That is, theadhesive is applied on the surface of a protective film for a polarizeror a PVA film that is a polarizer by using a roll coater, a gravurecoater, a bar coater, a knife coater, a capillary coater, or the like.Before the adhesive is completely dried, the protective film and apolarizing film are combined with each other by using heat pressing orpressing at normal temperature by means of a combination roll. When ahot melt type adhesive is used, a heat pressing roll must be used.

When a polyurethane adhesive is used, a polyurethane adhesive producedby using an aliphatic isocyanate compound which does not cause yellowingdue to light may be used. When a one- or two-liquid type dry laminateadhesive or an adhesive having relatively low reactivity in respect toisocyanate and a hydroxy group is used, a solution type adhesive whichis diluted with an acetate solvent, a ketone solvent, an ether solvent,or an aromatic solvent may be used. In this connection, it is preferablethat the adhesive have low viscosity of about 5000 cps or less.Preferably, the adhesive has excellent storage stability and lighttransmittance of 90% or more at a wavelength of about 400 to about 800nm.

A adhesive agent may be used as long as it shows sufficient cohesivepower. It is preferable that after the combination, an adhesive agent issufficiently cured by heat or UV light to show resulting mechanicalstrength as high as that obtained with an adhesive and the interfaceadhesion of the adhesive agent is so strong so that delamination ispossible only when one of both films bonded to each other is destroyed.

Specific examples of the adhesive agent may include natural rubber,synthetic rubber, or elastomer, a vinyl chloride/vinyl acetatecopolymer, polyvinyl alkyl ether, polyacrylate, modified polyolefinadhesive, having excellent optical transparency, and a curable adhesiveagent containing a curing agent such as isocyanate.

The manufactured polarizing plate may be used for the various purposes.Specifically, the polarizing plate may be applied to an image displaydevice such as a polarizing plate for liquid crystal displays (LCD) anda polarizing plate for preventing the reflection of the organic ELdisplay device. In addition, the optical film according to the presentinvention may be applied to a complex polarizing plate in which variousoptical layers such as various types of functional layers, for example,a retardation plate such as a λ/4 plate and a λ/2 plate, an opticaldiffusion plate, a viewing angle enlargement plate, a luminanceimprovement plate, and a reflection plate are combined with each other.

The polarizing plate may include an adhesive agent layer on at least oneside thereof so as to be easily applied to image display devices and thelike. In addition, the polarizing plate may further include a releasefilm on the adhesive agent layer in order to protect the adhesive agentlayer until the polarizing plate is applied to an image display device.

In addition, the present invention provides an electronic device thatincludes the optical film or the retardation film. The electronic devicemay be an image display device such as LCDs.

For example, the present invention provides an image display device thatincludes a light source, a first polarizing plate, a liquid crystalcell, and a second polarizing plate, which are sequentially stacked, andalso includes the optical film or the retardation film according to thepresent invention as at least one protective film of the firstpolarizing plate and the second polarizing plate or a retardation filmthat is provided between at least one of the first polarizing plate andthe second polarizing plate and the liquid crystal cell.

The liquid crystal cell includes a liquid crystal layer; a substratethat is capable of supporting the liquid crystal layer; and an electrodelayer to apply voltage to the liquid crystal. At this time, thepolarizing plate according to the present invention may be applied toall the liquid crystal modes such as an In-Plane Switching mode (IPSmode), a Vertically Aligned mode (VA mode), an OCB mode (OpticallyCompensated Birefringence mode), a Twisted Nematic mode (TN mode), aFringe Field Switching mode (FFS mode), and the like.

MODE FOR INVENTION

Hereinafter, the present invention will be described in more detail withreference to Examples. However, the following Examples are only forillustrative purposes and are not intended to limit the scope of thepresent invention.

EXAMPLES Example 1

A resin composition included 72 wt % of an acrylic resin having a weightratio of styrene-methylmethacrylate-maleic acid anhydride (SM-MMA-MAH)of 23:70:7 and a weight average molecular weight of about 130,000; and28 wt % of a core-shell type graft copolymer including a copolymer, as acore, of about 18 parts by weight of butadiene and about 3 parts byweight of methylmethacrylate, grafted by about 56 parts by weight ofmethylmethacrylate, about 20 parts by weight of styrene, and about 3parts by weight of cyclohexylmaleimide, and having a weight averagemolecular weight of about 100,000 and a graft ratio of about 40%. Theresin composition was dry-blended, and then a same direction biaxialextruder was used to prepare a heat-resistant blend in a pellet state.The prepared pellet was dried, and then an extruded film with athickness of about 80 μm (micrometer) was prepared by using an extruderincluding a T-dye. Characteristics of the film as a polarizer protectivefilm were measured, and the film was uniaxially stretched at Tg−10° C.in a transverse direction (TD) to prepare an IPS mode retardation film.Characteristics of the resulting film were measured, and the results areshown in the following Tables 1 and 2.

Example 2

A film was prepared in the same manner as in Example 1, except for usingabout 5 parts by weight of cyclohexylmaleimide of the core-shell typegraft copolymer, and then characteristics of the film were measured. Theresults are shown in the following Tables 1 and 2.

Example 3

A film was prepared in the same manner as in Example 1, except that thecore-shell type graft copolymer had a weight average molecular weight ofabout 150,000, and then characteristics of the film were measured. Theresults are shown in the following Tables 1 and 2.

Example 4

A film was prepared in the same manner as in Example 1, except that anacrylic resin having a weight ratio of styrene-methylmethacrylate-maleicacid anhydride-cyclohexylmaleimide (SM-MMA-MAH-CHMI) of 23:70:5:2 andhaving a weight average molecular weight of about 130,000 was used andabout 5 parts by weight of cyclohexylmaleimide was used in thecore-shell type graft copolymer, and then characteristics of the filmwere measured. The results are shown in the following Tables 1 and 2.

Comparative Example 1

A film was prepared in the same manner as in Example 1, except thatcyclohexylmaleimide was not included in the core-shell type graftcopolymer, and then characteristics of the film were measured. Theresults are shown in the following Tables 1 and 2.

Comparative Example 2

A film was prepared in the same manner as in Example 1, except thatcyclohexylmaleimide was not included in the core-shell type graftcopolymer and the copolymer had a weight average molecular weight ofabout 200,000, and then characteristics of the film were measured. Theresults are shown in the following Tables 1 and 2.

TABLE 1 Core-shell type graft copolymer Acrylic resin CHMI (part by CHMI(wt %) weight) Mw Example 1 0 3 100,000 Example 2 0 5 100,000 Example 30 3 150,000 Example 4 2 5 100,000 Comparative 0 0 100,000 Example 1Comparative 0 0 200,000 Example 2 CHMI: Cyclohexylmaleimide

TABLE 2 Retardation before Retardation after Coefficient of Haze, Filmsurface stretching uniaxial stretching T_(g) Thermal Expansion % state(R_(in)/R_(th)) (R_(in)/R_(th)) (° C.) (CTE, ppm/K) Example 1 0.8 ⊚1.2/2.0 100/45 120.4 78 Example 2 1.0 ⊚ 1.0/2.2 110/40 122.7 71 Example3 1.2 ◯ 0.8/2.0 110/37 121.0 76 Example 4 0.8 ⊚ 1.2/2.0 100/40 122.2 72Comparative 0.7 ⊚ 1.1/2.0 100/47 116.2 109 Example 1 Comparative 3.2 X1.2/2.4 110/45 118.5 97 Example 2 (1) Haze: measured in accordance withASTM 1003. (2) Mw (Weight Average Molecular Weight): measured bydissolving a pellet in tetrahydrofuran and subjecting the solution togel permeation chromatography (GPC). (3) Film surface state: The sizesand numbers of bubbles generated on a film surface were visuallymeasured when the film was pressed (⊚: Very good, ◯: Good, Δ: Fair, X:Poor). (4) Retardation value (R_(in)/R_(th)): measured by using aAxoScan from Axometrics, Inc. after stretching a film at glasstransition temperature. (5) Glass Transition Temperature (T_(g)):measured by using a Differential Scanning Calorimeter (DSC) from TAInstrument, Inc. (6) Coefficient of Thermal Expansion (CTE): measured ina temperature range of about 40° C. to about 90° C. by using aDifferential Scanning Calorimeter (DSC) from TA Instrument, Inc.

From the results in Tables 1 and 2, it can be seen that the optical filmaccording to the present invention may be used for various purposes suchas a polarizer protective film the like to replace an expensive TAC filmthat has been conventionally used because the film has excellent impactresistance and heat resistance and the film also has excellent opticalisotropic properties before stretching, and the optical film may be alsoused as a retardation film because the film may provide uniform andstable retardation after stretching.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

1. An optical film comprising: an acrylic resin, and a core-shell typegraft copolymer wherein the core comprises a conjugate diene rubber, andthe shell comprises an acrylic monomer, an aromatic vinyl monomer, and amaleimide monomer.
 2. The film of claim 1, wherein the acrylic resincomprises at least one selected from the group consisting of a homo- orcopolymer of an acrylic monomer; a copolymer of an acrylic monomer andan aromatic vinyl monomer; and a copolymer of an acrylic monomer, anaromatic vinyl monomer, and an acid anhydride.
 3. The film of claim 1,wherein the acrylic resin is a copolymer of an acrylic monomer, anaromatic vinyl monomer, and an acid anhydride, and a weight ratio ofeach monomer is (60˜98):(1˜45):(1˜15).
 4. The film of claim 3, whereinthe copolymer of an acrylic monomer, an aromatic monomer, and an acidanhydride comprises a maleimide monomer as an additional comonomer. 5.The film of claim 1, wherein the acrylic resin has a weight averagemolecular weight of about 100,000 to about 180,000.
 6. The film of claim1, wherein the conjugate diene rubber comprises at least one selectedfrom the group consisting of an ethylene-propylene diene rubber and abutadiene rubber.
 7. The film of claim 1, wherein the core of thecore-shell type graft copolymer comprises a copolymer of a conjugatediene rubber and an acrylic monomer.
 8. The film of claim 1, wherein theconjugate diene rubber is comprised in an amount of about 10 to about 60parts by weight based on 100 parts by weight of the core-shell typegraft copolymer in 2).
 9. The film of claim 1, wherein a weight ratio ofeach monomer of the copolymer comprising the acrylic monomer, thearomatic vinyl monomer, and the maleimide monomer in the shell is(38˜88):(1˜30):(1˜10).
 10. The film of claim 1, wherein the core-shelltype graft copolymer has a weight average molecular weight of about80,000 to about 300,000.
 11. The film of claim 1, wherein a weight ratioof the acrylic monomer and the core-shell type graft copolymer is(60˜99):(1˜40).
 12. The film of claim 1, wherein the optical film has aglass transition temperature of about 110° C. or more, a coefficient ofthermal expansion (CTE) of about 120 or less, and a haze of about 2.0%.13. A method of manufacturing an optical film, the method comprising:preparing a resin composition comprising an acrylic resin and acore-shell type graft copolymer wherein the core comprises a conjugatediene rubber, and the shell comprises an acrylic monomer, an aromaticvinyl monomer, and a maleimide monomer, and using the resin compositionto form the film.
 14. A retardation film comprising: an acrylic resin,and a core-shell type graft copolymer wherein the core comprises aconjugate diene rubber, and the shell comprises an acrylic monomer, anaromatic vinyl monomer, and a maleimide monomer.
 15. The retardationfilm of claim 14, wherein the retardation film has an in-planeretardation value of about 100 nm to about 120 nm, represented byMathematical Formula 1 and a thickness retardation value of about 20 nmto about 60 nm, represented by Mathematical Formula 2:R_(in)=(n _(x) −n _(y))×d  Mathematical Formula 1R_(th)=[(n _(x) +n _(y))/2−n _(z) ]×d,  Mathematical Formula 2 whereinn_(x) is a refractive index in a direction where the index is largest,in the film plane, n_(y) is a refractive index in a directionperpendicular to the n_(x) direction, in the film plane, n_(z) is arefractive index in a thickness direction, and d is film thickness. 16.A method of manufacturing a retardation film, the method comprising:preparing a resin composition including an acrylic resin and acore-shell type graft copolymer wherein the core comprises a conjugatediene rubber, and the shell comprises an acrylic monomer, an aromaticvinyl monomer, and a maleimide monomer, using the resin composition toform the film, and uniaxially or biaxially stretching the film.
 17. Apolarizing plate comprising: a polarizer; and a protective film providedon at least one side of the polarizer, wherein the protective film isthe optical film of claim
 1. 18. An electronic device comprising theoptical film of claim
 1. 19. An electronic device comprising theretardation film of claim
 14. 20. An electronic device comprising thepolarizing plate of claim 17.